The present invention relates to a driving power distribution apparatus and a method for controlling a torque coupling.
Conventionally, driving power distribution apparatuses equipped with a torque coupling have been known. A torque coupling is provided in a driving power transmission system that transmits torque of an engine to wheels. By adjusting the engaging force of a clutch mechanism, the torque coupling changes an amount of transmittable torque, that is, an amount of torque transmission. As one example of such a torque coupling, a torque coupling has been known that includes a first rotating member, which is formed like a cylinder, and a second rotating member, which is formed like a shaft and rotatably and coaxially arranged in the first rotating member. The torque coupling includes a clutch mechanism, which is located between the first rotating member and the second rotating member. The clutch mechanism couples the first rotating member and the second rotating member to each other such that torque can be transmitted therebetween (for example, refer to Japanese Laid-Open Patent Publication No. 2005-3167).
In each driving power transmission member that transmits torque of an engine to wheels (for example, a transfer case), the energy corresponding to transmission loss, which occurs when driving power is transmitted, is converted into heat and emitted. Thus, when the vehicle is traveling a certain condition, for example, when the vehicle continuously travels with a large amount of torque transmission, the driving power transmission member may be overheated. This can lead to seizing of the driving power transmission member.
Japanese Laid-Open Patent Publication No. 2002-349604 discloses another driving power distribution apparatus. This driving power distribution apparatus estimates the temperature of a driving power transmission member based on the amount of heat generated by the driving power transmission member generated by torque transmission and the ambient temperature of the driving power transmission member. When the estimated temperature is higher than or equal to a predetermined temperature, a control target value of the torque transmission amount of the driving power distribution apparatus is adjusted to be less than a case where the estimated temperature is lower than the predetermined temperature. This suppresses heat generation due to torque transmission at each driving power transmission member, and thus prevents the driving power transmission member from being overheated.
The temperature of each driving power transmission member is changed not only by heat generation through the torque transmission, but also by heat transfer from the engine. Particularly, in a traveling state where the engine load is high, for example, when the vehicle is towing another vehicle or when the vehicle is going uphill with the maximum number of occupants and the maximum load, the amount of heat transmitted from the engine to the driving power transmission member is increased.
However, according to the above described publication, the temperature of the driving power transmission member is estimated based on the amount of heat generated in the driving power transmission member and the ambient temperature of the driving power transmission member. Thus, in a driving state where the engine load is high, the temperature of the driving power transmission members might be estimated to be lower than the actual temperature.
When the vehicle is towing another vehicle or when the vehicle is going uphill with the maximum number of occupants and the maximum load, the high engine load condition lasts for an extended period. Therefore, a determination criterion set without considering such an extended period of high engine load may prevent control for reducing a control target value of the torque transmission amount from being carried out even if heat is accumulated in the driving power transmission member.
Thus, the conventional apparatus cannot sufficiently prevent overheating of driving power transmission members, and is susceptible to improvement in this respect.